Almost as old as the Fischer-Tropsch process for making hydrocarbons is the Fischer-Tropsch process for making alcohols. The reaction is carried out by passing a mixture of carbon monoxide and hydrogen over a catalyst for the hydrogenation of the carbon monoxide. A typical review article is R. B. Anderson et al., Industrial and Engineering Chemistry, Vol. 44, No. 10, pp. 2418-2424 (incorporated herein by reference). This paper lists a number of catalysts containing zinc, copper, chromium, manganese, thorium, iron, occasionally promoted with alkali or other materials for making various alcohols. The authors state that ethyl alcohol is a major constituent, the yield of methanol is usually very small and a tentative summary of factors favoring the production of alcohols is high pressure, low temperature, high space velocity, high recycle ratio and carbon monoxide-rich synthesis gas.
Molybdenum, tungsten and rhenium are known to be catalytic for the Fischer-Tropsch process. Murchison et al. in U.S. Pat. No. 4,151,190 (1979) and U.S. Pat. No. 4,199,522 (1980) (both incorporated herein by reference). The references describe some Fischer-Tropsch catalysts but do not teach that the catalyst is useful for making commercially significant quantities of alcohols. These references note that hydrogen sulfide affects the activity of the catalyst.
Pedersen et al., British Patent Publication No. 2,065,491 (incorporated herein by reference), disclose a process for making C.sub.2 hydrocarbons from H.sub.2 /CO using a catalyst comprising a group VB and/or VIB element in combination with an iron group metal as free metals, oxides or sulfides on a porous oxidic support. The authors note that the presence of H.sub.2 S alters the activity and selectivity of their process.
Chang et al., U.S. Pat. No. 4,177,202 (incorporated herein by reference), disclose a process for making hydrocarbons from H.sub.2 /CO over a molybdena and optionally cobalt or vanadium catalyst. Selectivity to ethane is enhanced by presence of hydrogen sulfide in the feed.
Stewart, U.S. Pat. No. 2,490,488 (incorporated herein by reference), discloses that molybdenum sulfide methanation catalysts acquire Fischer-Tropsch activity when promoted with an alkaline compound of an alkali metal. The example of the invention shows a 30 percent selectivity to C.sub.3 +hydrocarbons and oxygenates. Of this 30 percent, no more than 44 percent boils near or above 65.degree. C., the boiling point of methanol. Accordingly, the maximum possible alcohol selectivity is no more than 13.2 percent (44 percent of 30 percent).
Frankenburg, U.S. Pat. No. 2,539,414 (incorporated herein by reference), describes a Fischer-Tropsch process with molybdenum carbide catalysts. It teaches that the catalyst may be used to form oxygenates and at column 3, lines 66-71 teaches that one might get alcohols or hydrocarbons by varying the conditions.
Morgan et al., J. Soc. Chem. Ind., 51, pp. 1T-7T (Jan. 8, 1932), describe a process for making alcohols with chromium/manganese oxide catalysts promoted with alkali.
A number of references teach production of alcohols using rhodium catalysts. Some of these contain molybdenum as an optional ingredient. Ellgen et al., U.S. Pat. No. 4,014,913 (incorporated herein by reference), discloses a catalyst containing rhodium for the production of ethanol. Ellgen et al., U.S. Pat. No. 4,096,164 (incorporated herein by reference), discloses the use of rhodium in combination with molybdenum or tungsten. Example A discloses that use of a molybdenum-on-silica catalyst yielded 4.4 percent oxygenates. Ball, EPO application No. 81-33,212 (Chemical Abstracts 96:51,800a) (incorporated herein by reference), discloses a similar process using rhodium in combination with one or more of a long list of metals which includes molybdenum.
Hardman et al., EPO application No. 79-5,492 (Chemical Abstracts 92:166,257b) (incorporated herein by reference), disclose the production of alcohols using a 4-component catalyst. The first component is copper; the second is thorium; the third is an alkali metal promoter; and the fourth is a long list of metals one of which is molybdenum. Diffenbach et al., Chemical Abstracts 96:106,913x, disclose a nitrided iron catalyst which is promoted with molybdenum for making alcohols from synthesis gas.
Kinkade (Union Carbide), European Patent Application No. 84116467.6 (published July 24, 1985, Publ. No. 149,255) (incorporated herein by reference), discloses that C.sub.1-5 n-alcohols are substantially produced with a catalyst consisting essentially of molybdenum sulfide and an alkali metal compound. The gas hourly space velocity (i.e., GHSV) must be about 3000 hour.sup.-1 or above. Variations in the GHSV, temperature, pressure and alkali metal compound are disclosed to affect the alcohols' selectivity.
Quarderer et al. (Dow Chemical), European Patent Application No. 84102932.5 (published Sept. 26, 1984, Publ. No. 119,609) (incorporated herein by reference), discloses that alcohols which boil in the range of motor gasoline are made at good selectivities from syngas with an optionally supported Mo/W/Re and alkali/alkaline earth element catalyst. In certain preferred embodiments, it is disclosed that Mo/W/Re sulfides, carbon supports (when the catalyst is supported) are favored, and it is preferred to exclude cobalt.
It is known to dope certain catalysts with cobalt. However, no resultant selectivities to certain alcohols are taught to be incrementally and simply varied therewith. See, Shultz et al., U.S. Bureau of Mines, RI 6974 (incorporated herein by reference).
To make a commercially significant alcohol process, one must use a catalyst and conditions which are highly efficient. To be efficient, the catalyst must yield a high weight ratio of product per unit weight of catalyst in a given period of time. The catalyst must be stable and active for long periods of time between regenerations. This may be particularly difficult to accomplish when the H.sub.2 /CO ratio of the feed gas is low, such as less than 2 to 1. Ideally the catalyst will be sulfur tolerant and will have a high selectivity to a commercial product to avoid purification or removal and disposal of by-products and to avoid separation into two or more product streams.
When the mixed alcohols product is to be used as a fuel replacement or a fuel additive, it may be desirable that the ratio of C.sub.1 to C.sub.2 + alcohols be no greater than a certain amount. Excessive methanol is generally considered an unattractive additive to gasolines. Methanol may decrease drivability and may increase corrosion in the fuel system and may cause phase separation when used in excessive quantities. These problems may be alleviated by blending methanol with higher alcohols.
Accordingly, one may wish to synthesize mixed alcohols with no more than a certain amount of methanol in the blend. Or in a similar fashion, one may wish to select the ratio of C.sub.1 to C.sub.2 + alcohols in mixed alcohols so that methanol may be purchased and blended into the mixed alcohols to give the maximum acceptable C.sub.1 to C.sub.2 + alcohols ratio.
A problem in the art to be solved is enhancing the efficiency of the overall process to include catalyst preparation. Another problem to be solved is how to simply and efficiently select the ratio of C.sub.1 to C.sub.2 + alcohols without exclusively relying on distilling of the product stream, varying the sulfur content of the feed stream or without relying on mixing of various discrete catalysts. Another problem to be solved is the conservation of energy in the conversion into C.sub.2 + alcohols. Another is the usual requirement of a higher GHSV to obtain more C.sub.2 + alcohols with concurrent decrease in carbon monoxide conversion.